The present invention relates to an electrophotosensitive member, and more particularly to a photosensitive member having a plurality of amorphous silicon:germanium photoconductive layers.
Amorphous silicon:germanium (hereinafter referred to as a-Si:Ge), because of its small band gap as compared with amorphous silicon (hereinafter referred to as a-Si), shows a high absorption toward long wavelength light. Since, therefore, it contributes to the generation of many carriers and improvement in the sensitivity toward long wavelength light, it is expected to be used in the future as a photosensitive member for printers using semiconductor laser light. Also, since its sensitivity toward short-wave light is not damaged, it can be applied to plain paper copies (hereinafter referred to as PPC) by regulating the emission spectrum of exposure lamps. Also, a-Si:Ge has an excellent feature in that, because it absorbs long wavelength light, there is little disturbance of images by interference of light frequently encountered in the conventional amorphous silicon (a-Si) photosensitive members.
Because of these features, many studies for applying a-Si:Ge to photosensitive members are being made.
For example, there are disclosed a technique to use a-Si:Ge over the whole region of a photosensitive layer [Japanese Patent Application Kokai (Laid-open) No. 171038/1983]; a technique to apply a-si:Ge directly on a conductive base of a photosensitive member (U.S. Pat. No. 4,490,450); and a technique to apply a-Si:Ge to a layer in direct contact with the surface layer and/or substrate of a photosensitive member (ibid., No. 150753/1981). But, none of these techniques makes a proposal to apply a plural number of a-Si:Ge layers.
For example, said patent application No. 17-1038/1983 includes the formation of a-Si:Ge layer over the whole region of a photosensitive layer, but a-Si:Ge has its own defect in that it is small in .mu..tau. (carrier range) and low in carrier-carrying efficiency. When a-Si:Ge is therefore applied over the whole region of a photosensitive layer, generated carriers are trapped by the a-Si:Ge layer to cause not only reduction of sensitivity, but also generation of light fatigue and residual potential.
Also, as described in said U.S. Pat. No. 4,490,450 and in said Japanese Patent Application Kokai (Laid-open) No. 150753/1981, when the a-Si:Ge layer has been applied to the base of the photosensitive layer, because of a-Si:Ge being easy to generate thermally excited carriers, injection of charges at the base becomes easy to cause reduction of charging capability. Besides, when the thickness of the a-si:Ge layer is made large in order to eliminate interference patterns generated in printers using semiconductor laser ray or long-wave coherent light as a light source, carriers present in the vicinity of the base are trapped by the a-Si:Ge layer to cause reduction in sensitivity and generation of light fatigue and residual potential.
Further, as disclosed in Japanese Patent Application Kokai (Laid-open) No. 150753/1981, when the a-Si:Ge layer has been applied to the outermost surface of photosensitive members, carriers excited by short-wave light cannot migrate to move out of the layer to fail to contribute to sensitivity. While, when the thickness of the a-Si:Ge layer is made large in order to inhibit interference of light, carriers are trapped in the layer. Also, since a-Si:Ge generates a large number of thermally excited carriers, it causes injection of charges from the surface and this obviously lowers the charging capability.
For this reason, the foregoing conventional techniques do not make the best use of the excellent characteristics of a-Si:Ge.
On the other hand, Japanese Patent Application Kokai (Laid-open) No. 154850/1983 discloses an example of providing triple layers of a-Si:Ge to form the photosensitive member which has a photosensitivity extending to the long wavelength region. But the object of this photosensitive member is to control specific resistance and conductivity. This patent application does not refer at all to the use of a plural number of a-Si:Ge layers as a means for solving the problems encountered in using a-Si:Ge, i.e. a reduction in carrier-carrying efficiency accompanied by reduction of sensitivity and generation of light fatigue and residual potential.
As described above, a-Si:Ge, because of its small band gap as compared with a-Si, shows a high absorption toward long wavelength light, and therefore, it contributes to the generation of many carriers and improvement in the sensitivity toward long wavelength light.
But, the function of electrophotosensitive members does not work at all by mere doping of Ge. For example, high degrees of doping of Ge, when carried out randomly, increase the impurity level in the band gap to cause a reduction in the charging accepting capability which should be said to be the life of electrophotosensitive members. As a result, excellent electrostatic latent images are no longer obtained.
Also, since a-Si:Ge will increase the number of carriers generated but disturbs the movement of them, careless increasing of the amount of Ge added as well as the thickness of the a-Si:Ge layer makes the movement of carriers impossible, thereby causing the reduction of sensitivity, generation of residual potential, etc. In addition, because erasing is not also sufficiently attained, there occur very unfavorable results for electrophotography such as generation of memory etc.
On the other hand, in electrophotography with coherent light as a light source such as laser beam printers, etc., sufficient absorption of long wavelength light should be carried out in order to inhibit the generation of interference phenomenon.